Laser diodes are used to optically pump optical fiber (gain fiber), which has been doped to enable amplification of light signals. In common commercial products, 980 nanometer (nm) or 1480 mn diode lasers are used to optically pump erbium-doped fiber amplifiers operating or amplifying typically in a spectral range around 1550 nm.
In these diode pump-gain fiber systems, it is important to minimize changes in the amplifier characteristics due to changes in the pump wavelength or power. This is especially true in wavelength division multiplexing (WDM) systems or dense wavelength division multiplexing (DWDM) systems comprising many, spectrally closely-spaced channels. For example, mode hopping in the pump can cause changes in the gain spectrum of the amplifier. These changes result in preferential amplification of channels relative to other channels in the DWDM system.
One solution to controlling noise and wavelength shift due to environment temperature or power changes in the pumps uses fiber-grating stabilization. The Bragg rating has the effect of stabilizing the output spectrum from the laser pump or, more specifically, the grating stabilizes the pump against temporal power fluctuations. Further, in one suggested implementation, the grating is selected, spaced from the laser module, and tuned relative to the laser""s exit facet reflectivity so that the spectrum of the emission is broadened relative to that of a solitary laser.
To further stabilize pump lasers, polarization control is many times useful. The light emitted from the output facet of the diode lasers is typically highly polarized. The polarization of the light propagating through regular, non-polarization maintaining fiber, however, can change its orientation due to fiber birefringence, fiber twisting, bending, temperature shifts, and other stresses. Any fluctuation in the polarization of the light returning to the optical device from the grating effectively changes the feedback power ratio, because the laser is insensitive to any reflected light that has polarization orthogonal to that of the emitted light. For example, if all of the reflected light has its polarization rotated by 90 degrees, the fiber Bragg grating is effectively removed from the system from the standpoint of the laser.
In applications where polarization control is required between the laser diode and the grating, polarization-maintaining (PM) fiber is used for the fiber pigtail, with the grating being written into the PM fiber.
It should be appreciated, however, that the need for polarization control between optical devices and fiber gratings is not limited to pump lasers. These issues also concern general laser diodes or any fiber system (such as fiber amplifiers) with fiber grating stabilization for narrow line to coherence collapse operation and systems utilizing amplifiers such as Fabry-Perot lasers.
As a general rule, optical component manufacturers have resisted the use of polarization-maintaining fiber with fiber gratings. There are a number of justifications for this. Gratings are relatively hard to write in PM fiber, which impacts component cost. Further, narrowband gratings written in PM fiber will have reflectivity peaks at two discrete wavelengths, one for each polarization axis, because the birefringence results in a different effective grating pitch for each axis. This effect has an impact on operation if the source""s polarization is not aligned with the fast or slow axis of the fiber. Moreover, in many situations, the customer may not want to splice to PM fiber because of the high splice loss associated with fusing regular fiber to PM fiber in the field and/or expense associated with field deployment of sophisticated fusion splicing gear and the training required for the technicians.
Consequently, the use of PM fiber in fiber-grating stabilized optical systems is only prescribed when the disadvantages associated with PM fiber deployment outweigh problems associated with having no polarization control between the optical device and grating. For example, the mechanical rigidity and temperature stability of the optical system can be increased to thereby control stress-induced birefringence and consequently stabilize feedback from the grating into the diode laser, thus decreasing the need for polarization control. The long-term stability of these solutions, however, is unclear.
The present invention is directed to a solution for implementing gratings with optical sources where polarization-maintaining fiber is required or desirable.
Specifically, polarization-maintaining fiber is used between the grating and the optical source. The grating, however, is actually written in regular, or non-polarization-maintaining fiber. In one embodiment, the polarization-maintaining fiber is spliced directly to the non-PM fiber. Since this splice exists in the components, it can be performed in laboratory or production conditions, which preferably use a dedicated fusion splicer with controlled processes.
In general, according to one aspect, the invention features a fiber-grating stabilized optical component. This component comprises an optical source or system from which light is supplied. Depending on the implementation, optical sources or systems, such as lasers, specifically 980 nmxcx9c1480 run or Raman pump lasers, or amplifiers are used. Light from the optical source or system is transmitted through a polarization-maintaining fiber pigtail, which provides the desirable polarization control. Non-polarization-maintaining fiber is then coupled to the polarization-maintaining fiber, either directly or indirectly. The required grating is written into the non-polarization-maintaining fiber. The grating is used to affect the spectral characteristics of the light emitted from the optical component.
In the preferred embodiment, the non-polarization maintaining fiber is directly spliced to the polarization-maintaining fiber. In the current implementation, fusion splicing is used.
In the anticipated implementation, a module housing is used to contain the optical source. The polarization-maintaining fiber pigtail extends through a wall of this housing to terminate in proximity to the output facet of the laser source. Various techniques can be used to maximize the efficiency with which light from the optical source is coupled into the polarization-maintaining fiber, such as discrete lenses, butt coupling, and microlenses (formed or attached) at the end of the fiber pigtail.
Preferably, the grating is located within a distance of 1.0 or 0.50 meters, but typically the grating is less than 150 millimeters, from the junction between the polarization-maintaining fiber pigtail and the non-polarization-maintaining fiber. This ensures that the polarization control between the optical source and the grating is maximized.
In general, according to another aspect, the invention is also directed to a fibergrating-stabilized pump laser. This laser comprises a diode laser that generates light to optically-pump a fiber amplifier. A module houses the diode laser. A polarization-maintaining fiber pigtail extends through a wall of the module to terminate in proximity to an output facet of the diode laser to receive at least a portion of the light it generates. Non-polarization-maintaining fiber is optically coupled, directly or indirectly, to the polarization-maintaining fiber pigtail and a grating is written into the non-polarization-maintaining fiber to provide the desired polarization control.
Preferably, the grating has a power reflectivity of about 1.3% to 2.3%, preferably 1.4% to 2.0%. The front facet power reflectivity of the laser chip is 4.0% to 6.5%, preferably 4.5% to 6.0%. These ranges are used with a laser-to-fiber coupling efficiency of 70-75%.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.